In some jurisdictions, regulatory requirements are in force, or planned, that require mobile communication network operators to report the location of a mobile terminal of the network when the mobile makes a call to an emergency service bureau. In the United States, for example, the reported location is required to be accurate to 125 meters in 67% of the cases. Such systems therefore require a reliable manner of accurately determining the location of a mobile. Various other services that require knowledge of the location of a mobile are being provided or are in development.
One way to determine the location of a mobile in an ideal synchronous mobile radio communication system (i.e. one in which all base stations maintain the same or known transmission frequencies, and in which there is a known, stable timing relation between the beginning of transmissions, or of timing markers, transmitted from each base station) is through measurement of the time difference of arrival (TDOA) of radio signals from base stations of the network at the mobile. By measuring the difference in arrival times of radio signals from two base stations having a known timing relationship, the location of the mobile may be determined to lie somewhere along a hyperbolic surface (hyperboloid) between the base stations. If the TDOAs three base or more station pairs are known, the intersection of the hyperboloids defined by the TDOA measurement for each base station pair indicates the geographic position of the mobile. Alternatively, mobile location may be determined through measurement by three or more base stations of the TDOA of signals from a mobile.
The use of TDOA measurements for location determination is dependent upon precise measurements and precise synchronization of network elements. Time of arrival differences must be measured to sufficient resolution to locate a mobile with the desired position accuracy. A TDOA measurement or synchronization error of one microsecond will yield a positional error of approximately 300 meters. Present CDMA mobile communication systems, for example, systems based on the TIA standard IS-95, are designed to provide synchronization of signal timing to within a microsecond. However, this timing uncertainty and the resulting positional error still limits the practical applications of mobile TDOA location services.